The overall objective of the proposed research is to determine how the expression of a complex genetic locus, the white locus of Drosophila melanogaster, is regulated. First, we will determine the complete DNA sequence of the 13,000 base pair region which we have previously isolated and shown to contain the white locus. Second, we will characterize RNA transcripts of the locus with regard to their structure(s) and time and tissued specificity of synthesis. We will also determine the effect on white transcription of mutations at the zeste locus and in the pigment biosynthetic pathways. Third, we will determine what DNA sequences within the locus are required for pigment expression, proper developmental regulation, proper dosage compensation and epistatic interaction with the zeste locus using in vitro mutagenesis in conjunction with P-element-mediated DNA transformation. Fourth, we will attempt to determine the mechanism(s) by which these DNA sequences function to control the expression of the locus. Fifth, we will examine in detail the effects of varying the chromosomal location of the white locus on its expression. Sixth, we will investigate how certain abnormal white loci can produce non-clonal, but highly reproducible, patterns of pigmentation within the eye. The white locus has been an experimental system of choice for studying many intriguing genetic phenomena such as position effect variegation, dosage compensation, and transvection. A better understanding of the white locus itself should help elucidate these phenomena. Moreover, the white locus provides an escellent system to visualize and study non-clonal patterns. Non-clonal pigmentation patterns are widespread in nature. Non-clonal patterns of gene expression - the sensing of positional information within a developing organism - are fundamental to the process of metazoan development.